A significant portion of the current civil infrastructure is partially or completely constructed of cementitious materials, such as concrete. Cementitious materials are typically characterized as quasi-brittle materials with low tensile strength and low strain capacity. Fibers can be incorporated into cementitious matrices to overcome these weaknesses. Typical reinforcement of concrete is provided using reinforcing bars and macrofibers, both of which reinforce concrete on the millimeter scale. Recently, the use of microfiber reinforcement has led to significant improvement of the mechanical properties of cement based materials (Akkaya et al., ACI SP-216-1 2003, 216, 1-18). However, while microfibers delay the development of formed microcracks, they do not stop their initiation. The development of new nanosized fibers has opened a new field for nanosized reinforcement within concrete. The incorporation of fibers at the nanoscale according to the invention will allow the control of the matrix cracks at the nanoscale level and essentially create a new generation of a “crack free material”.
One of the most advantageous nanomaterials for nano-reinforcement are carbon nanotubes (CNTs). Since their discovery from Iijima (Iijima, Nature 1991, 354, 56-58), CNTs have opened an incredible range of applications in materials science, electronics, chemical processing, energy management, and many other fields. A CNT can be thought of as a sheet or sheets of graphite that have been rolled up into a tube structure. CNTs can be single walled nanotubes (SWCNTs), as if a single sheet had been rolled up with diameter close to 1 nm, or multiwalled (MWCNTs), similar in appearance to a number of sheets rolled together with diameter ranges from 10-80 nm. The unique mechanical, electrical and chemical properties of CNTs make them an attractive candidate for the next generation of composite materials. The Young's modulus of an individual nanotube should be around 1 TPa and its density is about 1.33 g/cm3 (Salvetat et al., Appl. Phys. A 1995, 69, 255-260). Compared to steel, CNTs possess five to ten times greater modulus than steel at just one sixth the weight. Molecular mechanics simulations suggested that CNTs fracture strains were between 10% and 15%, with corresponding tensile stresses on the order of 65 to 93 GPa (Belytschko et al., Phys. Rev. B 20052 65, 235430-235437). The aspect ratios of CNTs are generally beyond 1000.
Carbon nanotubes are expected to have several distinct advantages as a reinforcing material for cement as compared to more traditional fibers. First, they have significant greater strengths than conventional fibers, which strengths should improve overall mechanical behavior. Second, they exhibit higher aspect ratios, requiring significantly higher energies for crack propagation than would be the case for a lower aspect ratio fiber. Thirdly, CNTs exhibit smaller diameters which means that, provided that they are uniformly dispersed according to the invention, they can be widely distributed in the cement matrix with reduced fiber spacing. As illustrated by Akkaya et al. (ACI SP-216-1 2003, 216, 1-18), the tensile strength of the composite is increased when the fiber free area is decreased.
However, the potential of using nanotubes as reinforcement for cementitious materials has not been fully realized heretofore mainly because of the difficulties in processing. The two major drawbacks associated with the incorporation of CNTs in cement based materials are poor dispersion and cost. CNTs tend to adhere together due to Van der Waal forces, and it is particularly difficult to separate them individually (Groert, Materials Today 2007, 10, 28-35). To achieve good reinforcement in a composite, it is critical to have uniform dispersion of CNTs within the matrix. Poor dispersion of CNTs leads to the formation of many defect sites in the nanocomposite and limits the efficiency of the CNTs in the matrix (Xie et al., Mater. Sci. Eng. Rep. 2005, 49, 89-112).
Earlier attempts have been made to add CNTs in cementitious matrices at an amount ranging from 0.5 to 2.0 wt % (by weight of cement). Prior work on CNTs in liquid dispersions has focused on pre-treatment of the nanotube's surface via chemical modification. Makar et al. (Makar and Beaudoin, Proceedings of the 1st International Symposium on Nanotechnology in Construction, Royal Society of Chemistry 2004, 331-341; Makar et al., Proceedings of the 3rd International Conference on Construction Materials: Performance, Innovations and Structural Implications, Vancouver, B.C., Canada 2005, 1-10) reported an ethanol/sonication technique for dispersing 2.0 wt % CNTs in cement. The results obtained from SEM and Vickers hardness measurements indicate that CNTs affect the early hydration progress, producing higher hydration rates. Li et al. (Carbon 2005, 43, 1239-1245; Cem. Concr. Comp. 2007, 29, 377-382) employed a carboxylation procedure to improve the bonding between 0.5 wt % MWCNTs and cement matrix and obtained modest improvements in compressive and flexural strength. Saez de Ibarra et al. (Physica Status Solidi (a) 2006, 203, 1076-1081) used gum Arabic as a dispersing agent and reported modest gains in compressive strength and Young's modulus. Wansom et al. (Cem. Concr. Comp. 2006, 28 509-519) investigated the electrical properties of CNT-cement nanocomposites using a polycarboxylate based superplasticizer and methylcellulose with 0.75 and 0.1 vol % of CNTs. However, despite the efforts to date, only marginal success for nanotube reinforced cement based materials has been realized, mainly because of the above mentioned barrier to dispersion.